This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2003-327676 filed in Japan on Sep. 19, 2003 and Patent Application No. 2004-094078 filed in Japan on Mar. 29, 2004, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a static electricity protective circuit, and in particular, to a static electricity protective circuit for use in a high-frequency circuit apparatus such as a converter, an IF signal switching apparatus, or a reception apparatus for receiving satellite broadcasts.
2. Description of the Related Art
Among high-frequency apparatuses, there are a low-noise block down-converter (LNB) which is a reception apparatus for receiving satellite broadcasts, a receiver, and an IF signal switching unit (switch box) connected between the LNB and the receiver so as to select a signal from among IF (intermediate frequency) signals. Since a core conductor used for input and output terminals provided in these high-frequency apparatuses is directly connected to an internal circuit of the high-frequency apparatuses, devices such as transistors that constitute the internal circuit may be deteriorated or damaged, if static electricity is applied to the core conductor of the input and output terminals.
FIG. 13A is an exterior view of an ordinary LNB, and FIG. 13B is an exterior view of an ordinary switch box. FIG. 13C is an exterior view showing a connection between the ordinary LNB and an ordinary receiver. In these drawings, reference numeral 1 represents an LNB; reference numeral 2 represents a switch box; and reference numeral 3 represents a receiver such as, for example, a tuner or a television receiver.
The LNB 1 has a signal terminal 1a from which an intermediate-frequency signal (IF signal) is outputted. The switch box 2 has, for example, three connecting terminals (signal terminals) 2a to be connected to the LNB 1 and, for example, four connecting terminals (signal terminals) 2b to be connected to the receiver 3. It is to be noted that the number of connecting terminals 2a or 2b provided in the switch box 2 is not limited to these examples. It is also to be noted that, if the switch box 2 is not used, then, as shown in FIG. 13C, the signal terminal 1a of the LNB 1 is directly connected to a signal terminal 3a of the receiver 3 by a cable 4 such as a coaxial cable.
Because the signal terminal 1a of the LNB 1 and the connecting terminals 2a and 2b of the switch box 2 are exposed to the environment, static electricity built up externally may be applied to the core conductor of the signal terminal 1a or to the core conductors of the connecting terminals 2a or 2b when, for example, a connecting cable is connected thereto, or when the LNB 1 or the switch box 2 wrapped with such a material as vinyl which easily causes static building is unpacked. Since these core conductors are directly connected to the internal circuit of the LNB 1 or the switch box 2, devices such as transistors which constitute the internal circuit may be deteriorated or damaged, if static electricity is applied to these core conductors.
To prevent this from happening, the LNB 1 has a function of protecting the internal circuit thereof from static electricity. FIG. 14 is a circuit diagram showing a partial internal circuit of a conventional LNB. Reference numeral 1a represents the signal terminal which is to be connected to the receiver 3 shown in FIG. 13C. Through the signal terminal 1a, an IF signal of which a frequency is converted by the internal circuit of the LNB 1 is fed out and, at the same time, DC current is supplied from the receiver 3 as a power source for driving the LNB 1. To be more specific, the IF signal and the DC current are superimposed and supplied to the signal terminal 1a. For this reason, inside the LNB 1, an RF signal line through which the IF signal, i.e., an RF (radio-frequency) signal, is transmitted and a DC line through which the DC current is supplied, are separated from each other.
First, a configuration of an RF signal line side will be described. The signal terminal 1a is connected, through a ceramic capacitor 11, to one end of an attenuator 12 that performs an impedance matching. Other end of the attenuator 12 is connected to an RF amplifier 14 through a ceramic capacitor 13. The RF amplifier 14 amplifies a satellite broadcasting signal received by the LNB 1 and mixes the amplified signal with a locally oscillated signal so as to obtain and output an IF signal of which the frequency is converted.
Next, a configuration of a DC line side will be described. One end of a microstrip line 15 is connected to the signal terminal 1a and other end thereof is connected to ground through a by-pass capacitor 16. Moreover, the DC current that is supplied from the receiver 3 is extracted from a node between the microstrip line 15 and the capacitor 16 as a power source for the internal circuit of the LNB 1. At the same time, a surge absorber 17 is connected between the node and ground. Here, a line length of the microstrip line 15 is so adjusted as to be equal to a quarter of a wavelength of the frequency of the IF signal.
In this configuration, the IF signal transmitted from the RF amplifier 14 and the DC current supplied from the receiver 3 are superimposed with each other at the signal terminal 1a. Since DC components of the DC current are cut off by the capacitors 11 and 13, the DC current never flows through the attenuator 12 and the RF amplifier 14. Furthermore, the microstrip line 15 having a line length equal to a quarter of the wavelength of the frequency of the IF signal exhibits a low impedance to the DC current and current at a relatively low frequency, and exhibits a high impedance to the IF signal which is a high frequency signal. As a result of this, the IF signal never leaks through to the DC line or ground. To be more specific, transmission losses of the IF signal will not be caused.
If static electricity or a surge current is applied to the signal terminal 1a, the microstrip line 15 acts as a low-impedance conductor for the static electricity or the surge current, because most of frequency components of the static electricity or the surge current are DC components or AC components at relatively low frequencies. The surge absorber 17 also exhibits a low impedance to the static electricity or the surge current applied thereto. As a result of this, most part of the static electricity or the surge current applied thereto is dissipated to ground through the microstrip line 15 and the surge absorber 17, and the internal circuit of the LNB 1 will not be affected by the static electricity or the surge current. In this way, the LNB 1 is protected against the static electricity and the surge current.
Next, before explaining how the switch box 2 is protected against the static electricity or the surge current will be described, functions of the switch box 2 will be described. FIG. 15 is an exterior view showing an interconnection among the LNB, the switch box, and the receiver. FIG. 16 is an exterior view showing another interconnection among the LNBs, the switch box, and the receivers. In FIGS. 15 and 16, such components as are found also in FIGS. 13A to 13C are identified with the same reference numerals and descriptions thereof will not be repeated. FIG. 15 shows a case in which one LNB 1 and one receiver 3 are connected to the switch box 2. FIG. 16 shows a case in which three LNBs 1 and three receivers 3 are connected to the switch box 2. As shown in FIGS. 15 and 16, the switch box 2 is connected to the LNB(s) 1 and the receiver(s) 3 in such a way as to be placed in between them. In other words, in FIG. 16, each connecting terminal 2a of the switch box 2 is connected to the signal terminal 1a of each of the plurality of LNBs 1 independently by a cable 4a such as a coaxial cable. Also, each connecting terminal 2b of the switch box 2 is connected to the signal terminal 3a of each of the plurality of receivers 3 independently by a cable 4b such as a coaxial cable.
The switch box 2 to be connected in this way is a unit having a function as a switch for changing signal paths between the LNBs 1 and the receivers 3 so that an IF signal sent out from one LNB 1 can be received by the plurality of receivers 3, or a desired IF signal can be selected, at the receiver 3, from among the IF signals sent out from the plurality of LNBs 1. To achieve this, the switch box 2, according to a selection control signal transmitted from the receiver 3, selects the IF signal from among signals sent out from the plurality of LNBs 1 and leads the selected IF signal to a predetermined connecting terminal 2b. The selection control signal transmitted from the receiver 3 is fed from the signal terminal 3a of the receiver 3 to the connecting terminal 2b of the switch box 2 as a digital signal in pulses. DC current is supplied from the signal terminal 3a of the receiver 3 to the connecting terminal 2b of the switch box 2 as a power source for driving the switch box 2 and the LNB 1 connected thereto. Since the IF signal, the selection control signal, and the DC current are fed to and superimposed with one another at the connecting terminal 2b of the switch box 2, these signals are separated as AC components and DC components in the switch box 2. Moreover, the IF signal fed from the LNB 1 and the DC current to be supplied thereto are fed to and superimposed with each other at the connecting terminal 2a of the switch box 2.
FIG. 17 is a circuit diagram showing a partial internal circuit of the conventional switch box 2. Reference numeral 2b represents a connecting terminal to be connected to the receiver 3. An IF signal is transmitted to the connecting terminal 2b from the LNB 1 through a high-frequency switch circuit 23 which is one of the internal circuits. At the same time, DC current is supplied thereto from the receiver 3. As a result, the IF signal and the DC current are supplied to and superimposed with each other at the connecting terminal 2b. For this reason, inside the switch box 2, an RF signal line through which the IF signal, i.e., the RF signal, is transmitted and a DC line through which the DC current is supplied, are separated from each other. Here, because an internal circuit connected to the connecting terminal 2a of the switch box 2 is configured in the same manner as the internal circuit 23 connected to the connecting terminal 2b, the description thereof will be omitted.
First, a configuration of an RF signal line side will be described. The connecting terminal 2b is connected, through a ceramic capacitor 21, to one end of an attenuator 22 that performs an impedance matching. Other end of the attenuator 22 is connected to the high-frequency switch circuit 23. The high-frequency switch circuit 23 is a switch circuit for selecting the LNB 1 to be connected to the receiver 3 in accordance with the selection control signal transmitted from the receiver 3 or selecting the IF signal from the LNB 1 so as to distribute the selected signal to the plurality of receivers 3.
Next, a configuration of a DC line side will be described. One end of a choke coil 24 is connected to the connecting terminal 2b and other end thereof is connected to ground through a capacitor 25. The DC current that is supplied from the receiver 3 is extracted from a node between the choke coil 24 and the capacitor 25 as a power source for driving the switch box 2 and the LNB 1. In addition, a surge absorber 26 is connected between the node and ground.
In this configuration, the IF signal transmitted from the high-frequency switch circuit 23 and the DC current supplied from the receiver 3 are superimposed with each other at the connecting terminal 2b. Since DC components of the DC current are cut off by the capacitor 21, the DC current never flows through the attenuator 22 and the high-frequency switch circuit 23. Furthermore, since only the DC components or low-frequency components flow through the DC line by way of the choke coil 24, and the IF signal is cut off by the choke coil 24, the IF signal never leaks through to the DC line or ground. As a result, transmission loses of the IF signal will not be caused. In this example, to prevent the IF signal from leaking from the RF signal line to the DC line, the choke coil 24 is used as a high-frequency cut-off element in lieu of the microstrip line 15 shown in FIG. 14. The reason for this will be described below.
As described above, the DC current supplied from the switch box 2 serves as a power source for driving the switch box 2 and the LNB 1. Because of this reason, a level of the DC current flowing through the DC line of the switch box 2 will be amounting to a sum of current for driving the switch box 2 and current for driving the LNB 1. If a plurality of LNBs 1 or a plurality of receivers 3 are connected, then the level of the DC current increases. Furthermore, it is desired that the power supply line (DC line) be designed to withstand an overcurrent so that the switch box 2 will not be destroyed by the current flowing from the switch box 2 into the LNB 1 even if a power supply circuit of the LNB 1 connected to the switch box 2 is short-circuited. Because of this reason, the choke coil 24 having a larger current-carrying capacity than the microstrip line 15 is used so as to permit a larger amount of current to pass through the DC line.
Furthermore, protection of the internal circuit is also provided by arranging the chock coil 24 and the surge absorber 26 so as to permit static electricity and a surge current to flow therethrough to ground if the static electricity or the surge current is applied to the connecting terminal 2b. 
As another conventional technology, Japanese Utility Model Laid-Open No. H01-103300 discloses a static electricity protective device for protecting an internal circuit of an electronic control apparatus having an input/output connector against high-voltage static electricity. According to the disclosure, a conductor having a plurality of sharp-point projections that form gaps for allowing electrical discharge between the projections and each of connector terminal pins, is provided in an input/output connector case, wherein the conductor is grounded.
Japanese Patent Application Laid-Open No. 2001-257311 discloses an electrostatic protective circuit for protecting a semiconductor device. In the circuit, a series combination of a fuse and a capacitor is inserted between a signal terminal and an inner circuit.
International Patent Publication W00044049 discloses a circuit for protecting against static electricity, comprising a first power supply terminal to which a first voltage is applied, a second power supply terminal to which a second voltage lower than the first voltage is applied, a signal terminal to which a signal voltage lower than the first voltage and higher than the second voltage is applied, a first diode connected in forward direction between the first power supply terminal and the signal terminal, a second diode connected in forward direction between the signal terminal and the second power supply terminal, a third diode connected in reverse direction between the first power supply terminal and the signal terminal, and a fourth diode connected in reverse direction between the signal terminal and the second power supply terminal, wherein forward voltage drops caused across the first and second diodes individually are so set as to be higher than a driving voltage supplied between the first and second power supply terminals.
Japanese Patent Application Laid-Open No. H06-204407 discloses a diode element comprising a diffused layer formed on a substrate surface and an inner surface of a trench arranged in a semiconductor substrate, and a metal layer formed in the trench and on the semiconductor substrate.
Japanese Patent Application Laid-Open No. H04-053161 discloses a static electricity protective circuit for preventing an input circuit incorporating a switch from being destroyed by static electricity that is applied by an operator and flows through the input circuit when the switch is operated. The static electricity protective circuit comprises two diodes connected in series of which a node is connected to a terminal of the switch, and a resistor connected between the node and the input circuit.
Japanese Patent Application Laid-Open No. S63-105517 discloses a protection circuit against static electricity, having either a diode connected between a terminal and a positive power supply or a diode connected between the terminal and a negative power supply, and having a parasitic diode residing between the positive and negative power supplies, wherein at least one diode among the plurality of diodes shall be replaced with a plurality of diodes having different reverse dielectric strength.
Japanese Utility Model Laid-Open No. H07-033052 discloses adding a reverse current preventing diode into a high-level output side circuit of an output buffer and using a Zener diode as a static electricity protective diode for protecting an input buffer.
Japanese Patent Application Laid-Open No. 2000-245057 discloses an ultra high-frequency circuit and its static electricity protecting method, wherein a combination of a microstrip line having a line length equal to a quarter of a wavelength, a resistor, and a capacitor is used.
However, because the conventional LNB 1 shown in FIG. 14 and the conventional switch box 2 shown in FIG. 17 are so configured that the DC current flows through the microstrip line 15 or the choke coil 24, it is necessary to use the elements having a larger current-carrying capacity when a larger DC current is used. This prevents reductions in size and cost. Furthermore, when the choke coil 24 is used, depending on the frequency components of the static electricity, it is possible that the impedance thereof is increased and, thereby, the protection performance against the static electricity is hampered.
According to the conventional technology described in Japanese Utility Model Laid-Open No. H01-103300, when a high-frequency signal is fed to the connector pins, transmission losses are caused in the high-frequency signal because of a leakage thereof caused by a capacitance formed by the gap for allowing electrical discharges.
According to the conventional technology described in Japanese Patent Application Laid-Open No. 2001-257311, the electrostatic protective circuit effectively operates only when the semiconductor device remains unused before it is mounted. Once the semiconductor device is mounted, the fuse in the electrostatic protective circuit is blown off, making the protective function against the static electricity inoperative.
According to the conventional technology disclosed in International Patent Publication W00044049, when a high-frequency signal is fed to the signal terminal, the high-frequency signal leaks to the first and second power supply terminals through each of the first to the fourth diodes, causing transmission losses of the high-frequency signal.
Since the diode element described in Japanese Patent Application Laid-Open No. H06-204407 is a Schottky barrier diode that occupies a smaller area and can be integrated in high packaging densities, it is possible to configure a static electricity protective circuit capable of operating at high speed while taking up a smaller area if such a diode element is used for the static electricity protective circuit. However, if such a protective circuit with the diode element alone is used for a high-frequency signal circuit, the high-frequency signal leaks, causing transmission losses thereof.
According to the conventional technology described in Japanese Patent Application Laid-Open No. H04-053161, the power supply line to which the two diodes are connected for leading the static electricity applied to the input circuit and the power supply line for the input circuit belong to different systems. Furthermore, the circuit is designed by excluding a possibility of a high-frequency signal being applied to the input circuit. For this reason, if a high-frequency signal is fed to the input circuit, transmission losses in the high-frequency signal are caused by the two diodes.
According to the conventional technology described in Japanese Patent Application Laid-Open No. S63-105517, it is necessary to provide one of positive and negative power supplies to which a diode for leaking the static electricity applied to the terminal is connected, and, therefore, it is impossible to protect a terminal to which a DC signal as a power supply and a high-frequency signal are superimposed with each other and fed. In addition, the high-frequency signal leaks through the diode, causing transmission losses in the high-frequency signal.
According to the conventional technology described in Japanese Patent Application Laid-Open No. H07-033052, the reverse current protection diode is for preventing a current caused by a difference between an output level and a reception level for receiving it, from flowing in reverse direction, and is irrelevant to the protection against static electricity. Moreover, because the static electricity protective diode for protecting the input buffer is placed between an input signal line and ground, if an input signal is a high-frequency signal, it is possible that the high-frequency signal leaks to ground due to a capacitance parasitic in the static electricity protective diode.
The static electricity protecting method used in the conventional technology described in Japanese Patent Application Laid-Open No. 2000-245057 provides effective protection against the static electricity for a high-frequency circuit. However, a combination of a resistor, a microstrip line, a capacitor, and two diodes is required for one terminal to protect against static electricity applied thereto. This requires an increased number of components for such apparatuses as an LNB of which a miniaturization is demanded. Because an ordinary diode is used, it is also possible that the diode itself may be damaged depending on the nature of static electricity applied thereto.